The present invention relates to a system for transporting high-viscosity materials; and more specifically, an efficient, progressing cavity pump system for transporting high-solids, high-viscosity, dewatered materials, such as dewatered sludge.
Sludge dewatering is one of the fastest growing segments of the municipal wastewater treatment industry. Municipal wastewater treatment plants who have previously placed their waste activated sludge in sludge lagoons or drying beds, or who have previously directly land-applied their waste activated sludge, are now being forced by EPA Section 503 regulations and economics to further process the sludge. Such further processing includes dewatering of the sludge.
EPA Section 503 regulations went into effect in 1993 to establish requirements for the final use and disposal of sewage sludge. These regulations escalated the costs of final disposal of sewage sludge, which in turn gave strong incentive to municipal wastewater treatment plants to reduce the amount of sludge being disposed. The removal of water from the sludge (dewatering) is one of the more practical means to reduce sludge volumes and waste. Therefore, municipal wastewater treatment plants have been using increasing amounts of resources to install or create more efficient dewatering equipment.
Presently, dewatered sewage sludge is transported away from dewatering devices by four different methods: belt conveyors, screw conveyors, piston pumps, and progressing cavity pumps. Pumps have inherent advantages over conveyors. Specifically, the sludge is transported through a pipeline, rather than being exposed to the atmosphere, which significantly reduces odor; sludge can fall off, or be blown off, a conveyor belt, causing a safety and house-cleaning problem; and a dewatered sludge pipeline is easy to heat-trace and insulate as opposed to conveyors, which are not possible to heat-trace or insulate.
In North America, piston pumps comprise a majority of the market share for dewatered sludge pumps. The most common piston pumps utilize a pair of material cylinders in which a corresponding pair of material pistons reciprocate. The sludge material is received at the inlets of each of the material cylinders, the feed into which is controlled by inlet gate tube or poppet valves. Additionally, the flow of the sludge from the material cylinders to an outlet is controlled by outlet poppet valves, respectively. The inlet valves are controlled by hydraulic inlet valve cylinders, and the outlet valves are, likewise, controlled by hydraulic outlet valve cylinders. The material pistons are coupled to hydraulic drive pistons, which are in turn operated according to a hydraulic control system. As the drive pistons and their associated material pistons come to an end of their stroke, one of the material cylinders is discharging material to the outlet, while the other material sender is loading material from the inlet. Accordingly, the inlet and outlet valves are controlled to allow the material to be discharged from the first cylinder and new material to be loaded into the second cylinder. At the next pump cycle, when the second material piston is at the end of its stroke, the inlet and outlet valves will be controlled such that the material is permitted to be discharged from the second cylinder and new material is permitted to be loaded into the first material cylinder. As a result of this design, the material in each cylinder will come to a stop each time a piston reaches the end of its stroke, allowing the inlet and outlet valves to change positions. This material must be then accelerated from a rest condition by a piston on the next stroke. Accordingly, significantly high pressure levels are generated in each of these cylinders during the stroke. Also, significantly high pressure levels are generated in the pipeline to overcome the resulting acceleration losses. Additionally, the resulting flow of materials from the outlet is a pulsating flow. A further disadvantage of the piston-type pumps is that the pumps must be powered by hydraulics and corresponding hydraulic and valve controls, which significantly increase the costs of the pump. Such complexity also increases the costs in maintenance and repairs for the pump systems.
Furthermore, according to federal regulations Section 503, municipal wastewater treatment plants are also required to measure and document the mass flow rate for sludge transport applications. In regard to incinerators, control efficiencies and sludge feed rates have to be reported in mass flow for the proper calculations in determining pollutant limits. A significant disadvantage with the use of the piston-type pump is that the determination of mass flow rate based on volume is complex due to the number of parameters needed for such calculations. For example, a hydraulically-driven piston pump requires two position switches in the hydraulic cylinder to sense the start and stop positions of the piston and to determine the stroke length. A third proximity switch on the discharge valve senses when the valve opens and closes. The piston pump must calculate the volumetric efficiency for each stroke of the pump. The stroke volume is large and even when being fed by a twin screw feeder, the volumetric efficiency could vary a significant amount between strokes, since the inlet valves are an obstruction to suction flow.
The volumetric efficiency is calculated by timing from a startup of the piston stroke to the opening of the valve, and timing from the opening of the valve to the end of the piston stroke when the valve closes. Using time instead of stroke position to determine volumetric efficiency does not compensate for fluctuations in velocity of the piston (i.e., the point where the piston actually goes from no-load to load). Accordingly, typical accuracy for such flow-rate calculation has been found to have a relatively high variance.
Progressing cavity pumps provide an alternative to piston pumps. A progressing cavity pump includes an elongated, externally-threaded rotor rotating within an elongated, internal helical-threaded stator, where the stator has one more lead or start than the rotor. Pumps of this general type are typically built with a rigid metallic rotor and a stator, which is formed from a flexible or resilient material such as rubber. The rotor is made to fit within the stator bore with a compressive fit, which results in seal lines where the rotor and stator contact. These seal lines define or seal off definite cavities bounded by the rotor and stator surfaces. As the rotor turns within the stator, the cavities formed by the seal lines progress from the suction end of the rotor/stator pair to the discharge end of the rotor/stator pair. During one revolution of the rotor, one set of cavities is opened at exactly the same rate that the second set of cavities is closing. This results in a predictable, pulsationless flow.
While such progressing cavity pumps are less expensive and less complicated than the piston pumps, conventional progressing cavity pump systems also have several characteristics that may make them less attractive for use in transporting the high-solids dewatered sludge. Specifically, the volumetric efficiency (filling efficiency) for conventional progressing cavity pumps in such applications can be approximately 50%. Additionally, the footprint of conventional progressing cavity pumps and associated feeders are relatively long and narrow, making it substantially difficult for most municipal wastewater treatment plants to be retrofitted with such systems.
Accordingly, there is a need for a pump system for transporting high-solids, high-viscosity, dewatered materials that is relatively inexpensive and uncomplicated, that has a compact design (footprint), that produces a non-pulsating flow, that has a relatively high volumetric efficiency, that allows for accurate and uncomplicated calculation of mass flow-rate, and that does not necessitate relatively high pressure levels within the system.
The present invention provides a system and method for transporting high-viscosity, high-solids, dewatered materials. The system essentially comprises a progressing cavity pump system utilizing a twin-screw feeder with an extended tunnel section. The feeding of the material into an extended tunnel section of the twin screw feeder creates a positive pressure, which assists in feeding the product into the suction housing of the progressing cavity pump, and correspondingly, into the pumping elements. This increases volumetric (fill) efficiency of the progressing cavity pump, thereby allowing a smaller pump to be used, and in turn, reducing the expense for the wastewater treatment facility.
The feeder mechanism of the present system is radially set apart from the progressing cavity elements, where the materials are transported from the extended tunnel section of the feeder to the suction housing of the progressing cavity pump by a transition conduit. In one embodiment, the feeder is positioned above the progressing cavity pumping elements providing a taller system but with a relatively small footprint. In another embodiment, the feeder is positioned along side the progressing cavity pumping elements, which reduces the height of the system but increases the width. In yet another embodiment, the feeder is positioned substantially perpendicular to the pump axis. While this embodiment provides the widest footprint, it will also provide the best flow transition of the materials.
The suction housing of the progressing cavity pump includes an auger positioned therein that is directly coupled to, and preferably integral with, the progressing cavity rotor. The universal joint is moved from the position in front of the stator entrance to a point behind the auger and the suction inlet to improve flow of material from the suction housing to the progressing cavity pump elements. The inlet conduit coupled to the transition housing is angled slightly towards the stator entrance to further improve the flow efficiency and increase the fill rate of the progressing cavity pump elements.
Optionally, the system will also include a lubrication injection ring positioned in the discharge section to decrease the friction between the product and the discharge pipe wall. This, in turn, decreases the amount of head pressure that the progressing cavity pump need to develop. The decrease in head pressure allows a smaller pump to be used and also decreases the maintenance time/cost of the system and energy consumed by the system.
The system also utilizes a simplified method that directly measures mass flow rate per revolution of the pump element. The calibration of the unit takes into consideration the volumetric and mechanical efficiency of the progressing cavity pump. Without obstruction of any inlet valves, the volumetric efficiency is constant and repeatable for a progressing cavity pump. With proper operation, mass flow rate calculations for this system will provide increased repeatability and accuracy.
The twin screw feeder of the present system maintains a consistent feed pressure into the progressing cavity pump, and in conjunction with the auger feed rotor in the suction housing of the progressing cavity pump, insures high volumetric efficiency for consistent pumping. The volumetric efficiency is dependent upon pump RPM and solid content, and when sized properly, the progressing cavity pumping elements combined with the twin screw feeder will consistently approach 100% volumetric efficiency.
A large diaphragm pressure sensor positioned in the suction housing of the progressing cavity pump monitors the inlet pressure to the pump. The sensor provides a signal to the feedback control module which then controls the speed of the twin screw feeder to maintain the optimal infeed pressure. A weight sensor in the twin screw feeder provides a signal to the control module, which will adjust the speed of the pump to maintain a constant sludge level in the twin screw feeder. By maintaining a constant amount of sludge in the feeder, the pump flow rate is matched to the rate of the belt press or centrifuge feed feeding the inlet hopper to the twin-screw feeder. A tachometer feedback on the pump drive registers the RPM and total quantity of pump revolutions for the production run. A discharge pressure sensor registers the discharge pressure for consistency indications. Data is also recorded and displayed through the control module.
Accordingly, it is an aspect of the present invention to provide a progressing cavity pump system that comprises: (a) an elongated progressing cavity pump having a suction housing, a discharge port, an elongated progressing cavity stator positioned between the suction housing and the discharge port, and an elongated progressing cavity rotor positioned for rotation within the progressing cavity stator; (b) a feeder having an elongated feeder housing, an inlet, an outlet at a longitudinal end of the feeder housing, and an auger mechanism positioned in the feeder housing for feeding material from the inlet to the outlet, where the elongated feeder housing is positioned radially apart from the elongated progressing cavity pump; and (c) a transfer conduit coupled between the outlet of the feeder and the suction housing of the progressing cavity pump. By positioning the feeder radially apart from the progressing cavity pump, the overall length of the pump system is decreased. This allows more municipal wastewater treatment facilities with limited room for such pumping systems to now utilize the more efficient, more robust, less complicated and less expensive progressing cavity pumps, as opposed to piston pumps.
In certain embodiments, the elongated feeder housing extends substantially parallel to the elongated progressing cavity pump. In one of such embodiments the elongated feeder housing is mounted on the frame extending over the progressing cavity pump, where the inlet of the feeder is an elongated opening extending into the top of the feeder housing, communicating with the hopper positioned above the opening.
It is preferred that the transfer conduit includes an outlet segment directly coupled to the suction housing of the progressing cavity pump and the outlet segment of the transfer conduit is angled at least partially away from the discharge port of the progressing cavity pump, thereby providing a substantially smooth transition for material being pumped from the transfer conduit and through the suction housing of the progressing cavity pump.
It is also preferred that the auger mechanism includes a pair of parallel, intermeshing, counter-rotating augers extending substantially the entire length of the feeder housing cavity, and the inlet to the feeder housing is positioned in the top of the feeder housing and extends from the longitudinal end of the feeder housing opposite the outlet end and to a point substantially distal from the outlet, and providing an extended tunnel section in the feeder approximate the outlet end of the feeder housing. Preferably, the extended tunnel section extends for at least two pitch lengths of the auger conveyor utilized by the auger mechanism. This extended tunnel section promotes a slight build-up pressure at the outlet end of the feeder, which assists in the volumetric efficiency to the progressing cavity pump elements. Additionally, a narrowing preload conduit is positioned between the outlet of the feeder and the suction housing of the progressing cavity pump. This narrowing conduit placed before the progressing cavity pump elements also promotes a pressure increase in the materials being fed, which further assists in the volumetric efficiency to the progressing cavity pump elements.
It is yet another aspect of the present invention to provide progressing cavity pump comprising: (a) an elongated stator housing having a suction end and a discharge end; (b) an elongated progressing cavity stator mounted within the stator housing; (c) an elongated progressing cavity rotor mounted for rotation within the progressing cavity stator, the progressing cavity rotor having a suction end and a discharge end; (d) a suction housing coupled to the stator housing at the suction end of the stator housing, the suction housing including an inlet port; (e) an auger positioned in the suction housing, directly coupled to and integral with the suction end of the progressing cavity rotor, where the auger includes a forward longitudinal end approximate the progressing cavity rotor and a rear longitudinal end distal from the progressing cavity rotor; and (f) a drive shaft extending into the suction housing having a forward longitudinal end and a rear longitudinal end, where the forward longitudinal end of the drive shaft is coupled to the rear longitudinal end of the auger by a universal joint. By moving the universal joint behind the behind the inlet and out of the pumpage in the suction housing, a smoother transition from the auger to the progressing cavity pump elements is provided, thus substantially improving the volumetric efficiency of the progressing cavity pump elements, and, in-turn, the mass flow rate of the system.
Preferably, the inlet port opening is positioned in a radial side wall of the suction housing, where the inlet port opening has a forward edge approximate the forward longitudinal end of the auger and a rear edge approximate the rear longitudinal end of the auger. The universal joint is preferably positioned behind the rear edge of the inlet port opening so that it is positioned substantially out of the flow path of materials through the suction housing. It is also preferred that the auger is fixedly coupled to the progressing cavity rotor and where the progressing cavity rotor has a diameter substantially equal to the diameter of the auger shaft so that a substantially smooth transition is provided from the auger shaft to the rotor. It is also preferred that the inlet conduit feeding the suction housing is angled at least partially rearward with respect to the auger, thereby providing an even smoother transition of material from the inlet conduit and through the suction housing.
It is also preferred that the progressing cavity pump includes a material feeder and fluid communication with the inlet conduit, where the material feeder includes a feeder housing, an inlet, an outlet at an end of the feeder housing, and an auger mechanism positioned in the feeder housing for feeding material from the feeder inlet to the feeder outlet. This feeder housing is preferably positioned radially apart from the suction housing, where the feeder housing may be positioned over top of the progressing cavity pump elements, or on the side of the progressing cavity pump elements to provide the system with a more compact design as discussed above.
It is also preferred that the auger mechanism of the feeder is positioned within an elongated cavity within the feeder housing and the feeder outlet is in fluid communication with an outlet of the elongated cavity; the auger mechanism of the feeder includes a pair of parallel, intermeshing augers positioned with the elongated cavity of the feeder and rotating in opposite directions, where the augers extend substantially the entire length of the elongated cavity within the feeder housing; and that the inlet to the feeder housing is positioned in the top of the feeder housing, radially adjacent to the auger mechanism, and extends from a longitudinal end of the elongated cavity within the feeder housing, opposite the outlet end, to a point substantially distal from the outlet end of the feeder cavity, providing an extended tunnel section within the feeder cavity at the outlet end of the feeder cavity. This tunnel section preferably extends for at least two pitches of the augers extending therethrough. As discussed above, this tunnel section within feeder promotes a pressure increase in the materials being fed to the progressing cavity pump elements, which improves volumetric efficiency.
Furthermore, it is preferred that the progressing cavity pump further includes a drive motor coupled to the rear longitudinal end of the drive shaft and a drive motor housing mounted to the suction housing, where the drive shaft is a hollow drive shaft.
It is yet another aspect of the present invention to provide a progressing cavity pump system that comprises: (a) a feeder mechanism including, (1) a feeder housing having in inlet, an outlet on an end of the feeder housing and an elongated cavity within the feeder housing, where the feeder outlet is in fluid communication with the elongated cavity, and (2) a pair of parallel, intermeshing augers positioned in the elongated cavity and rotating in opposite directions; (b) at least two progressing cavity pumps, each progressing cavity pump including a suction housing, an inlet in the suction housing, a discharge port, an elongated progressing cavity stator positioned between the suction housing and the discharge port, and an elongated progressing cavity rotor positioned for rotation within the progressing cavity stator; and (c) a transfer conduit coupled between the feeder outlet and the suction housing inlet of each of the progressing cavity pumps. This configuration provides controlled and uninterrupted flow of materials to more than one discharge point, such as in a multiple hearth incinerator, where several injection points around the cylindrically shaped furnace results in a controlled bum of the sludge. Other split-flow applications also exist, such as delivering sludge evenly along the length of a tractor trailer.
It is yet another aspect of the present invention to provide a method for transporting high-solids, dewatered materials that comprises the steps of: (a) introducing the materials into a hopper; (b) depositing the materials from the hopper to a pair of intermeshing, counter rotating augers in a feeder; (c) conveying the materials, by the augers, to an enclosed chamber (enclosed on all radial sides) within the feeder cavity; (d) generating a predetermined pressure increase in the enclosed chamber; (e) transporting the materials from the enclosed chamber to a suction port of a progressing cavity pump; and (f) pumping the materials, by the progressing cavity pump, to a discharge outlet.
Preferably, the conveying, transporting and pumping steps occur continuously, thereby, not allowing the material to stop moving between the feeder and the desired outlet. It is also preferred that the method includes the step of positioning the feeder in a location radially set apart from the progressing cavity pump. It is also preferred that the method further includes the steps of sensing the pressure of the material approximate the suction port of the progressing cavity pump, and controlling the speed of the feeder augers according to the pressure sensor reading. It is also preferred that the method includes the steps of sensing the amount of material present in the feeder cavity, and controlling the speed of the pump according to this reading. Preferably, the amount of material in the feeder cavity is sensed by a weight (or load) sensor in the feeder.
The method may also include the steps of positioning a lubrication source in a discharge section of the progressing cavity pump, and injecting lubrication, by the lubrication source, between the materials in the discharge section and the discharge section conduits. Preferably, these steps further include the step of sensing a pressure in the discharge section, and controlling the amount of lubrication injected by the lubrication source according to the pressure sensed in the discharge section.
It is yet another aspect of the present invention to provide a method for transporting high-solids, dewatered materials that comprises the steps of: (a) transporting the materials from a feeder to a suction port of a progressing cavity pump, the feeder having a feeder cavity and a feed mechanism positioned within the feeder cavity; (b) pumping the materials, by the progressing cavity pump, to a discharge outlet; (c) sensing a pressure of the material in a material path approximate the suction port of the progressing cavity pump; (d) controlling the speed of the feed mechanism according to the pressure sensed in step (c); (e) sensing an amount of material present in the feeder cavity; and (f) controlling the speed of the progressing cavity pump according to the amount of material present in the cavity as sensed in step (e). Preferably, the sensing step (e) includes the step of sensing a weight of the material in the feeder cavity.